Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection.
There are several different classes of OCT, but Fourier domain OCT currently offers the best performance for many applications. Moreover, of the Fourier domain approaches, swept-source OCT has distinct advantages over techniques such as spectrum-encoded OCT because it has the capability of balanced and polarization diversity detection. It has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for spectrum-encoded OCT, are not available.
In swept source OCT, the spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive optical frequency sampling intervals and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its frequency sweep rate and tuning accuracy.
High speed frequency tuning, or sweep rates, for OCT swept sources is especially relevant to in vivo imaging where fast imaging reduces motion-induced artifacts and reduces the length of the patient procedure. It can also be used to improve resolution.
The swept sources for OCT systems have typically been tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. A tunable laser is constructed from a gain medium, such as a semiconductor optical amplifier (SOA) that is located within a resonant cavity, and a tunable element such as a rotating grating, grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest tuning speed/sweep rate lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled Laser with Tilted Multi Spatial Mode Resonator Tuning Element, by D. Flanders, M. Kuznetsov and W. Atia. The use of micro-electro-mechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low mass, high mechanical resonant frequency deflectable MEMS membranes that have the capacity for high speed tuning/sweep rates. Another laser architecture is termed a Fourier-domain mode-locked laser (FDML). This type of laser stores light in a long length of fiber for amplification and recirculation in synchronism with the laser's tuning element. See “Fourier Domain Mode Locking (FDML): A new laser operating regime and applications for optical coherence tomography”, R. Huber, M. Wojtkowski, and J. G. Fujimoto, 17 Apr. 2006/Vol. 14, No. 8/OPTICS EXPRESS 3225. The drawback of these devices is their complexity, however. Moreover, the ring cavity including the long storage fiber creates its own performance problems such as dispersion and instability.
Another class of swept sources that has the potential to avoid inherent drawbacks of tunable lasers is filtered amplified spontaneous emission (ASE) sources that combine a broadband light source, typically a source that generates light by ASE, with tunable filters and amplifiers.
Some of the highest speed devices based on filtered ASE sources are described in U.S. Pat. No. 7,061,618 B2, entitled Integrated Spectroscopy System, by W. Atia, D. Flanders P. Kotidis, and M. Kuznetsov, which describes spectroscopy engines for diffuse reflectance spectroscopy and other spectroscopic applications. A number of variants of the filtered ASE swept source are described, including amplified versions and versions with tracking filters.
More recently Eigenwillig, et al. have proposed a variant configuration of the filtered ASE source in an article entitled “Wavelength swept ASE source”, Conference Title: Optical Coherence Tomography and Coherence Techniques IV, Munich, Germany, Proc. SPIE 7372, 73720O (Jul. 13, 2009). The article describes an SOA functioning both as an ASE source and first amplification stage. Two Fabry-Perot tunable filters are used in a primary-tracking filter arrangement, which are followed by a second SOA amplification stage. Also, U.S. patent application Ser. No. 12/553,295, filed on Sep. 3, 2009, entitled Filtered ASE Swept Source for OCT Medical Imaging, by D. Flanders, W. Atia, and M. Kuznetsov (U.S. Pat. Pub. No. US 2011/0051148 A1), which is incorporated herein in its entirety by this reference, lays out various integrated, high speed filtered ASE swept source configurations. U.S. patent application Ser. No. 12/776,373, filed on May 8, 2010, entitled ASE Swept Source with Self-Tracking Filter for OCT Medical Imaging, by the same inventors (U.S. Pat. Pub. No. US 2011/0051143 A1), outlines still further configurations that rely on the use of a self-tracking filter arrangement that can improve performance both in terms of sweep rate and linewidth, among other things, and which is also incorporated herein in its entirety by this reference.
In order to compensate for instabilities and/or non-linearities in the tuning of the OCT swept sources, a sampling clock (k-clock) is often employed to enable sampling at equally spaced increments in the optical frequency domain (k-space). This k-clock must usually be delayed to match the delay associated with the optical signals in the sample and reference arms of the interferometer of the OCT system.
If a k-clock is not used but the swept source tunes non-linearly, other corrective options are employed. Some resample the data equally in k-space by interpolation, see S. Yun, G. Tearney, B. Bouma, B. Park, and J. de Boer, “High-speed spectral-domain optical coherence tomography at 1.3 μm wavelength,” Opt. Express 11, 3598-3604 (2003).
Resampling in k-space, however, has disadvantages. This is because another metric that characterizes the performance of OCT systems is the electronic bandwidth of the electronic signal processing systems. Sufficiently high bandwidth is becoming increasingly important as higher speed, performance and resolution OCT systems are produced. For example, increasing the wavelength tuning speed of the swept source, which produces higher OCT image acquisition speeds, also results in greater requirements for the electronics that are used to sample the resulting optical interference signals. Typically, to accurately resample, oversampling must be employed, which adds overhead to the electronic signal processing systems.
In newer designs, the k-clock system is integrated with the swept source. An example is disclosed in U.S. patent application Ser. No. 12/396,099, filed on Mar. 2, 2009, entitled Optical Coherence Tomography Laser with Integrated Clock, by Flanders, et al. (U.S. Pat. Pub. No. US 2009/0290167 A1), which is incorporated herein by this reference. Here, the delay in the k-clock is provided electronically. This solution has certain advantages in that the electronic delay can be programmable to match changes in the interferometer delay that might be concomitant with the use of different OCT probes, for example.